1. Field of the Invention
The present invention relates to a laminated piezoelectric element and its production method and polarization process. The present invention also relates to a vibration wave driven motor.
2. Related Background Art
A rod-shaped vibration wave driven motor is already known and examples can be found in Japanese Patent Publication Nos. 3-40767 and 3-289375. FIG. 10 is an exploded perspective view of a vibrator for use in a rod-shaped vibration wave driven motor and FIG. 11 is a longitudinal sectional view of a rod-shaped vibration wave driven motor.
The vibrator shown in FIG. 10 has a piezoelectric element of phase A (a1) comprising two sheets of piezoelectric elements PZT1 and PZT2, a piezoelectric element of phase B (a2) comprising two sheets of piezoelectric elements PZT3 and PZT4, and a sensor piezoelectric element s1 comprising a disk of piezoelectric material, all of these being laminated in a manner as shown in FIG. 10. This vibrator also includes electrode disks A1 and A2 for supplying electric power across these piezoelectric elements and an electrode disk S for outputting a sensor signal. There are also provided electrode disks G1, G2, and G3 for GND connection. These piezoelectric elements and electrode disks are held between metal blocks b1 and b2 made of material such as brass and stainless steel showing relatively small damping. These metal blocks are fastened with a fastening bolt c to form a single piece. As a result of this, compressive stress is introduced in the piezoelectric elements. Because an insulating disk d is disposed between the bolt c and the metal block b2, additional sensor piezoelectric elements other than s1 are not required.
In this arrangement, the piezoelectric elements of phase A (a1) and phase B (a2) are disposed in such a manner that phase difference of 90.degree. exists between these elements. These piezoelectric elements a1 and a2 each excite bending vibrations in the directions of two planes, respectively, wherein each plane includes the axis of the vibrator and these two planes are perpendicular to each other. Furthermore, an adequate amount of time lag is introduced between two piezoelectric elements a1 and a2. As a result of these, portions of the surface of the vibrator move along a circular or ellipsoidal line of locus points. The movement of these portions friction-drives a moving element which is pressed onto the top of the vibrator.
FIG. 11 shows an example of a rod-shaped vibration wave driven motor having a vibrator of such a type as that described above. In this example, the fastening bolt c of the vibrator includes a supporting rod c2 having a smaller diameter at its end portion. A fastening element g is provided at the end portion of this supporting rod c2 so that motor itself is fastened with this fastening element g. This fastening element g also acts as a supporting element for rotating elements such as a rotor r. The rotor r is in contact with the top surface of the front metal block b1. A coiled spring h in a spring case i inserted into the rotor r is pressed by the fastening element g via a bearing element e and via a gear f, thus pressure is applied to the rotor r.
Referring now to FIG. 12, a the piezoelectric element for use in this rod-shaped vibration wave driven motor will be described in more detail hereinbelow. Each of piezoelectric disks PZT1-PZT4 is made by machining a sheet of piezoelectric ceramic obtained by sintering power material into a disk shape. The thickness is made 0.5 mm. Two electrode films 8-1 and 8-2 are formed on the top surface, wherein each electrode film has a semicircular shape and each is separated by a slit from the other. The whole of the back surface is covered with an electrode film 10. Then, polarization is performed by applying opposite voltages to semicircular electrode films 8-1 and 8-2 wherein the polarities are made different between right and left electrode films (+, -). Thus, piezoelectric characteristics are obtained. For example, in the phase A, piezoelectric elements PZT1 and PZT2 obtained in this way are disposed one on the other in such a manner that the same polarity of polarization is opposed to each other via the electrode disk A1 and that at the same time the slits are located at the same position. In the phase B, the piezoelectric elements are disposed in a similar manner. That is to say, the polarization direction of the piezoelectric element a1 of the phase A is such that as shown by the arrows 14 in FIG. 13. When the electrode disk A1 is applied with a driving AC voltage, one of piezoelectric elements PZT1 and PZT2 located on the right and left sides, respectively, is expanded and the other one is contracted. The expansion and contraction occur alternately, thus bending vibration occurs in the vibrator. In the phase B, bending vibration occurs in the same manner except that the direction of the slits is different from that of the phase A by 90.degree..
However, in the conventional arrangement described above, after the piezoelectric elements are polarized one by one, then they are stacked in such a manner that electrode disks and piezoelectric elements are disposed alternately. Therefore, it requires an extremely long time to perform polarization and also to assemble a vibrator. If the handling is considered of the piezoelectric element during the polarization or assembling, the thickness of the piezoelectric element should be larger than a certain value. Therefore, the stacking of further layers is difficult, because the total size would become large. This is a barrier to be overcome for achieving a smaller sized vibration wave driven motor having a size similar to that of a pencil or less than that. This is also a reason why it is difficult to achieve a high power vibration wave driven motor with a larger number of layers. For the same reason, it is difficult to achieve a low voltage driving operation.
Thus, it is an object of the present invention to provide a laminated piezoelectric device which needs only a short time for polarization.
It is another object of the present, invention to provide a novel method of polarization for a laminated piezoelectric device.
It is a further object of the present invention to provide a vibration wave driven motor which needs only a short time for assembling.
It is still another object of the present invention to provide an extremely small sized vibration wave driven motor.
It is another object of the present invention to provide a high power vibration wave driven motor.